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Abstract:

The present disclosure relates generally to an optical element, a light
projector that includes the optical element, and an image projector that
includes the optical element. In particular, the optical element provides
an improved uniformity of light by homogenizing the light with lenslet
arrays, such as "fly-eye arrays" (FEA). The FEA is positioned to
homogenize an unpolarized combined light before the light is converted to
a single polarization state.

Claims:

1. An optical element, comprising: a first lenslet array having a first
plurality of lenses disposed to accept an unpolarized light and output a
convergent unpolarized light; a second lenslet array having a second
plurality of lenses disposed to accept the convergent unpolarized light
and output a divergent unpolarized light; and a polarization converter
disposed to accept the divergent unpolarized light and output a polarized
light, wherein the first lenslet array and the second lenslet array are a
monolithic array, and an unpolarized light ray coincident with the
optical axis of a first lens of the first plurality of lenses is
coincident with the optical axis of a second lens of the second plurality
of lenses.

2. The optical element of claim 1, wherein the monolithic array comprises
a glass, a polymer, or a silicone.

3. The optical element of claim 1, wherein the monolithic array comprises
a polymeric material having a birefringence of less than about 50 nm at a
nominal wavelength of 550 nm.

4. The optical element of claim 1, wherein the unpolarized light ray is
split into a first polarized light ray and a second polarized light ray
having equal optical path lengths through the polarization converter.

5. The optical element of claim 1, wherein the monolithic array has a
thickness between about 2 mm and about 10 mm.

6. The optical element of claim 1, wherein the focal point of each of the
first plurality of lenses is positioned at a first principle plane of the
second plurality of lenses.

7. The optical element of claim 1, wherein the monolithic array further
comprises a polymer film disposed between the first plurality of lenses
and the second plurality of lenses.

8. The optical element of claim 1, wherein the first plurality of lenses
and the second plurality of lenses have a one-to-one correspondence.

9. The optical element of claim 1, wherein at least one of the first
plurality of lenses and the second plurality of lenses comprise
cylindrical lenses.

10. The optical element of claim 1, wherein at least one of the first
plurality of lenses and the second plurality of lenses comprise bi-convex
lenses, spherical lenses, or aspherical lenses.

11. The optical element of claim 1, wherein each of the first plurality
of lenses and each of the second plurality of lenses have a positive
power.

18. A light projector, comprising: a first unpolarized light source and a
second unpolarized light source; a color combiner disposed to output a
combined unpolarized light from the first unpolarized light source and
the second unpolarized light source; an optical element, comprising: a
first lenslet array having a first plurality of lenses disposed to accept
the combined unpolarized light and output a convergent unpolarized light;
a second lenslet array having a second plurality of lenses disposed to
accept the convergent unpolarized light and output a divergent
unpolarized light; and a polarization converter disposed to accept the
divergent unpolarized light and output a polarized light, wherein the
first lenslet array and the second lenslet array are a monolithic array,
and an unpolarized light ray coincident with the optical axis of a first
lens of the first plurality of lenses is coincident with the optical axis
of a second lens of the second plurality of lenses.

19. An image projector, comprising: a first unpolarized light source and
a second unpolarized light source; a color combiner disposed to output a
combined unpolarized light from the first unpolarized light source and
the second unpolarized light source; an optical element, comprising: a
first lenslet array having a first plurality of lenses disposed to accept
the combined unpolarized light and output a convergent unpolarized light;
a second lenslet array having a second plurality of lenses disposed to
accept the convergent unpolarized light and output a divergent
unpolarized light; a polarization converter disposed to accept the
divergent unpolarized light and output a polarized light; wherein the
first lenslet array and the second lenslet array are a monolithic array,
and an unpolarized light ray coincident with the optical axis of a first
lens of the first plurality of lenses is coincident with the optical axis
of a second lens of the second plurality of lenses; a spatial light
modulator disposed to impart an image to the polarized light; and
projection optics.

Description:

[0001] This application is related to the following U.S. patent
applications, which are incorporated by reference: "Compact Optical
Integrator" U.S. Ser. No. 61/292,574 (Attorney Docket No. 65902US002)
filed on Jan. 6, 2010; and also "Polarized Projection Illuminator"
(Attorney Docket No. 66249US002) and "Fly Eye Integrator Polarization
Converter" (Attorney Docket No. 66247US002), both filed on an even date
herewith.

BACKGROUND

[0002] Projection systems used for projecting an image on a screen can use
multiple color light sources, such as light emitting diodes (LED's), with
different colors to generate the illumination light. Several optical
elements are disposed between the LED's and the image display unit to
combine and transfer the light from the LED's to the image display unit.
The image display unit can use various methods to impose an image on the
light. For example, the image display unit may use polarization, as with
transmissive or reflective liquid crystal displays.

[0003] Still other projection systems used for projecting an image on a
screen can use white light configured to imagewise reflect from a digital
micro-mirror (DMM) array, such as the array used in Texas Instruments'
Digital Light Processor (DLP®) displays. In the DLP® display,
individual mirrors within the digital micro-mirror array represent
individual pixels of the projected image. A display pixel is illuminated
when the corresponding mirror is tilted so that incident light is
directed into the projected optical path. A rotating color wheel placed
within the optical path is timed to the reflection of light from the
digital micro-mirror array, so that the reflected white light is filtered
to project the color corresponding to the pixel. The digital micro-mirror
array is then switched to the next desired pixel color, and the process
is continued at such a rapid rate that the entire projected display
appears to be continuously illuminated. The digital micro-mirror
projection system requires fewer pixelated array components, which can
result in a smaller size projector.

[0004] Image brightness is an important parameter of a projection system.
The brightness of color light sources and the efficiencies of collecting,
combining, homogenizing and delivering the light to the image display
unit all affect brightness. As the size of modern projector systems
decreases, there is a need to maintain an adequate level of output
brightness while at the same time keeping heat produced by the color
light sources at a low level that can be dissipated in a small projector
system. There is a need for a light combining system that combines
multiple color lights with increased efficiency to provide a light output
with an adequate level of brightness without excessive power consumption
by light sources.

[0005] Such electronic projectors often include a device for optically
homogenizing a beam of light in order to improve brightness and color
uniformity for light projected on a screen. Two common devices are an
integrating tunnel and a fly's eye homogenizer. Fly's eye homogenizers
can be very compact, and for this reason is a commonly used device.
Integrating tunnels can be more efficient at homogenization, but a hollow
tunnel generally requires a length that is often 5 times the height or
width, whichever is greater. Solid tunnels often are longer than hollow
tunnels, due to the effects of refraction.

[0006] Pico and pocket projectors have limited available space for light
integrators or homogenizers. However, efficient and uniform light output
from the optical devices used in these projectors (such as color
combiners and polarization converters) can require a compact and
efficient integrator.

SUMMARY

[0007] The present disclosure relates generally to an optical element, a
light projector that includes the optical element, and an image projector
that includes the optical element. In particular, the optical element
provides an improved uniformity of light by homogenizing the light with
lenslet arrays, such as "fly-eye arrays" (FEA). In one aspect, the
present disclosure provides an optical element that includes a first
lenslet array having a first plurality of lenses disposed to accept an
unpolarized light and output a convergent unpolarized light. The optical
element further includes a second lenslet array having a second plurality
of lenses disposed to accept the convergent unpolarized light and output
a divergent unpolarized light. The optical element still further includes
a polarization converter disposed to accept the divergent unpolarized
light and output a polarized light. The first lenslet array and the
second lenslet array are a monolithic array, and an unpolarized light ray
coincident with the optical axis of a first lens of the first plurality
of lenses is coincident with the optical axis of a second lens of the
second plurality of lenses.

[0008] In another aspect, the present disclosure provides a light
projector that includes a first unpolarized light source and a second
unpolarized light source, a color combiner disposed to output a combined
unpolarized light from the first unpolarized light source and the second
unpolarized light source and an optical element. The optical element
includes a first lenslet array having a first plurality of lenses
disposed to accept the combined unpolarized light and output a convergent
unpolarized light, a second lenslet array having a second plurality of
lenses disposed to accept the convergent unpolarized light and output a
divergent unpolarized light, and a polarization converter disposed to
accept the divergent unpolarized light and output a polarized light. The
first lenslet array and the second lenslet array are a monolithic array,
and an unpolarized light ray coincident with the optical axis of a first
lens of the first plurality of lenses is coincident with the optical axis
of a second lens of the second plurality of lenses.

[0009] In yet another aspect, the present disclosure provides an image
projector that includes a first unpolarized light source and a second
unpolarized light source, a color combiner disposed to output a combined
unpolarized light from the first unpolarized light source and the second
unpolarized light source, an optical element, a spatial light modulator
disposed to impart an image to the polarized light, and projection
optics. The optical element includes a first lenslet array having a first
plurality of lenses disposed to accept the combined unpolarized light and
output a convergent unpolarized light, a second lenslet array having a
second plurality of lenses disposed to accept the convergent unpolarized
light and output a divergent unpolarized light, and a polarization
converter disposed to accept the divergent unpolarized light and output a
polarized light. The first lenslet array and the second lenslet array are
a monolithic array, and an unpolarized light ray coincident with the
optical axis of a first lens of the first plurality of lenses is
coincident with the optical axis of a second lens of the second plurality
of lenses.

[0010] The above summary is not intended to describe each disclosed
embodiment or every implementation of the present disclosure. The figures
and the detailed description below more particularly exemplify
illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Throughout the specification reference is made to the appended
drawings, where like reference numerals designate like elements, and
wherein:

[0016] The figures are not necessarily to scale. Like numbers used in the
figures refer to like components. However, it will be understood that the
use of a number to refer to a component in a given figure is not intended
to limit the component in another figure labeled with the same number.

DETAILED DESCRIPTION

[0017] This disclosure generally relates to image projectors, in
particular image projectors improve the uniformity of light by
homogenizing the light with lenslet arrays, such as "fly-eye arrays"
(FEA). In one particular embodiment, a compact polarized illumination
system includes a polarization converting system (PCS) and a molded
monolithic Fly-Eye Array (FEA) integrator. Combination of a polarization
converter with a monolithic FEA can result in both a high efficiency and
good uniformity simultaneously, in a compact system. The FEA integrator
includes arrays of convex lenses molded on two opposing surfaces.

[0018] LCoS-based portable projection systems are becoming common due to
the availability of low cost and high resolution LCoS panels. A list of
elements in an LED-illuminated LCoS projector may include LED light
source or sources, optional color combiner, optional pre-polarizing
system, relay optics, PBS, LCoS panel, and projection lens unit. For
LCoS-based projection systems, the efficiency and contrast of the
projector is directly linked to the degree of polarization of light
entering the PBS. For at least this reason, a pre-polarizing system that
either utilizes a reflection/recycling optic or a polarization-conversion
optical element is often required.

[0019] Polarization conversion schemes utilizing polarizing beam splitters
and half-wave retarders are one of the most efficient ways to provide
polarized light into the PBS. One challenge with polarization-converted
light is that it may suffer from spatial nonuniformity, leading to
artifacts in the displayed image. Therefore, in systems with polarization
converters, a homogenization system is desirable.

[0020] It is common in conventional projection systems that a FEA
consisting of a pair of thin glass microlenslet array plates separated by
an air gap is used to homogenize the light. In handheld projectors, such
a paired FEA system typically has the drawbacks of having greater
thickness and more challenging alignment tolerances.

[0021] More recently, single-element monolithic molded plastic or glass
FEA units have been adopted for very compact projection systems. However,
such molded monolithic units typically have maximum birefringence of 50
nm or more and high variation in retardance and optical axis orientation
and as such are only used for homogenizing unpolarized light. In some
cases, a single monolithic element with low birefringence following a
high-efficiency polarization converter can achieve high optical
efficiency, good image uniformity, and compact size simultaneously.

[0022] In one particular embodiment, an illuminator for an image projector
includes a light source in which emitted unpolarized light is directed
into a polarization converter. The polarization converter separates the
light into two paths, one for each polarization state. The path length
for each of the two polarization states is approximately equal, and the
polarized beams of light pass through to a monolithic FEA integrator. The
monolithic FEA integrator can cause the light beams to diverge, and the
light beams are then directed for further processing, for example, by
using a spatial light modulator to impart an image to the light beams,
and projection optics to display the image on a screen.

[0023] In some cases, optical projectors use a non-polarized light source,
such as a light emitting diode (LED) or a discharge light, a polarization
selecting element, a first polarization spatial modulator, and a second
polarization selecting element. Since the first polarization selecting
element rejects 50% of the light emitted from the non-polarized light
source, polarization-selective projectors can often have a lower
efficiency than non-polarized devices.

[0024] One technique of increasing the efficiency of
polarization-selective projectors is to add a polarization converter
between the light source and the first polarization selecting element.
Generally, there are two ways of designing a polarization converter used
in the art. The first is to partially collimate the light emitting from
the light source, pass the partially collimated beam of light through an
array of lenses, and position an array of polarization converters at each
focal point. The polarization converter typically has a polarizing beam
splitter having polarization selective tilted film (for example MacNeille
polarizer, a wire grid polarizer, or birefringent optical film
polarizer), where the reflected polarization is reflected by a tilted
mirror such that the reflected beam propagates parallel to the beam that
is transmitted by the tilted polarization selective film. Either one or
the other beams of polarized light is passed through half-wave retarders,
such that both beams have the same polarization state.

[0025] Another technique of converting the unpolarized light beam to a
light beam having a single polarization state is to pass the entire beam
of light through a tilted polarization selector, and the split beams are
conditioned by mirrors and half-wave retarders such that a single
polarization state is emitted. Illuminating a polarization selective
spatial light modulator directly with a polarization converter can result
in luminance non-uniformity and color non-uniformity.

[0026] In some cases, the polarization converter can be positioned after
the illumination light leaves a fly's eye array (FEA) homogenizing
component. In some cases, the polarization converter can be positioned in
front of an FEA homogenizing component, such that the illumination light
is polarized as it enters the FEA homogenizing component. One drawback of
the latter configuration is that the fly's eye array needs to be made
birefringence free, or at the least, a very low birefringence. It can be
challenging to control the molding fabrication process of the fly's eye
array with sufficient precision, in order to produce low birefringence
material. A much wider range of materials can be used, for example,
higher birefringence materials become acceptable, such as those having a
birefringence of about 50 nm or more, when the FEA homogenizing component
is placed after the illumination source and before the light is
polarized.

[0027] Generally, the FEA serves to homogenize the polarized illumination
light on the imager plane. Each of the pairs of lenses on opposite
surfaces of the FEA spread the light over the imager plane, such that the
illumination light is effectively blended. The light bundle sampled by a
first lenslet on the first FEA surface is focused by a second lenslet on
the second FEA surface. The light is then redistributed subsequent optics
to cover the entire imager plane, such as an LCoS imager plane. This
process is repeated across the FEA for each of the lenslet pairs, so that
even if the light distribution is non-uniform in the front of the FEA, it
will be redistributed to form a uniform light distribution on the imager.

[0028] In one particular embodiment, a polarization converter can
incorporate a fly's eye array to homogenize the light in a projection
system. The input side of the polarization converter includes a
monolithic FEA to homogenize the light. The input and output side of the
monolithic FEA include the same number of lenses, with each lens on the
output side centered approximately at the focal point of a matching lens
at the input side. The lenses can be cylindrical, bi-convex, spherical,
or aspherical; however, in many cases spherical lenses can be preferred.
The fly's eye integrator and polarization converter can significantly
improve the illuminance and color uniformity of the projector.

[0029] The lenses of the monolithic FEA may be fabricated by
microreplicating plastic lenses on a first film, which can be cut,
aligned, and bonded to microreplicated plastic lenses on a second film.
Another alternative is to mold one or both lenslet arrays as single units
out of glass or plastic, and bond those together without an intervening
film. The lenslet arrays may be made from a single axis lens, such as a
cylindrical lens or a lens with two axes of refraction, such as a
spherical lens. The number of lenses on each of the input and output
surfaces of the monolithic FEA may range from a single lens, a single
dimensional array of lenses, to a two dimensional array of lenses. In one
particular embodiment, each of the input and output surfaces of the
monolithic FEA can include a rectangular array of spherical lenses, such
as a square array having a size ranging from a 5×5 array to a
20×20 array or more. Generally, a larger array of lenses can reduce
the separation between the arrays, so that the overall size of the
projection system can be reduced.

[0030] In some cases, a folded fly eye array can homogenize the
illuminating light. A folded fly-eye array can be formed with a first
lenslet array, a folding mirror, and a second lenslet array, where the
lenses making up the second lenslet array are approximately at the focal
point of the lenses making up the first lenslet array.

[0031] FIG. 1 shows a schematic diagram of an image projector 100,
according to one aspect of the disclosure. Image projector 100 includes a
color combiner module 110 that is capable of injecting a combined light
output 124 into a homogenizing polarization converter module 130 where
the combined light output 124 becomes converted to a homogenized
polarized light 145 that exits the homogenizing polarization converter
module 130 and enters an image generator module 150. The image generator
module 150 outputs an imaged light 165 that enters a projection module
170 where the imaged light 165 becomes a projected imaged light 180.

[0033] In one aspect, the received inputs light sources 112, 114, 116, are
unpolarized, and the combined light output 124 is also unpolarized. The
combined light output 124 can be a polychromatic combined light that
comprises more than one wavelength spectrum of light. The combined light
output 124 can be a time sequenced output of each of the received lights.
In one aspect, each of the different wavelength spectra of light
corresponds to a different color light (for example red, green and blue),
and the combined light output is white light, or a time sequenced red,
green and blue light. For purposes of the description provided herein,
"color light" and "wavelength spectrum light" are both intended to mean
light having a wavelength spectrum range which may be correlated to a
specific color if visible to the human eye. The more general term
"wavelength spectrum light" refers to both visible and other wavelength
spectrums of light including, for example, infrared light.

[0034] According to one aspect, each input light source (112, 114, 116)
comprises one or more light emitting diodes (LED's). Various light
sources can be used such as lasers, laser diodes, organic LED's (OLED's),
and non solid state light sources such as ultra high pressure (UHP),
halogen or xenon lamps with appropriate collectors or reflectors. Light
sources, light collimators, lenses, and light integrators useful in the
present invention are further described, for example, in Published U.S.
Patent Application No. US 2008/0285129, the disclosure of which is herein
included in its entirety.

[0035] In one aspect, homogenizing polarization converter module 130
includes a polarization converter 140 that is capable of converting
unpolarized combined light output 124 into homogenized polarized light
145. Homogenizing polarization converter module 130 further can include a
monolithic array of lenses 101, such as a monolithic FEA of lenses
described elsewhere that can homogenize and improve the uniformity of the
combined light output 124 that exits the homogenizing polarization
converter module 130 as homogenized polarized light 145.

[0037] Suitable spatial light modulators (that is, image generators) have
been described previously, for example, in U.S. Pat. Nos. 7,362,507
(Duncan et al.), 7,529,029 (Duncan et al.); in U.S. Publication No.
2008-0285129-A1 (Magarill et al.); and also in PCT Publication No.
WO2007/016015 (Duncan et al.). In one particular embodiment, homogenized
polarized light 145 is a divergent light originating from each lens of
the FEA. After passing through imaging optics 152, 154 and PBS 156,
homogenized polarized light 145 becomes imaging light 160 that uniformly
illuminates the spatial light modulator. In one particular embodiment,
each of the divergent light ray bundles from each of the lenses in the
FEA illuminates a major portion of the spatial light modulator 158 so
that the individual divergent ray bundles overlap each other.

[0038] In one aspect, projection module 170 includes representative
projection optics 172, 174, 176, that can be used to project imaged light
165 as projected light 180. Suitable projection optics 172, 174, 176 have
been described previously, and are well known to those of skill in the
art.

[0039]FIG. 2 shows a side-view schematic of an optical element 200,
according to one aspect of the disclosure. Optical element 200 can be
used as the homogenizing polarization converter module 130 in the image
projector 100 as shown in FIG. 1. Optical element 200 includes a first
lenslet array 210, a second lenslet array 230, and a polarization
converter 220. Each of the first lenslet array 210 and the second lenslet
array 230 can be referred to as a "Fly-Eye Array", or FEA, as known in
the art. In some cases, each of the first lenslet array 210 and the
second lenslet array 230 can include a converging (that is, positive)
power. The first lenslet array 210 and the second lenslet array 230
together form a monolithic FEA 201 that has a thickness "t", and can
include an optional central substrate 214 between first lenslet array 210
and second lenslet array 230. Generally, the thickness "t" can be about
10 mm, about 6 mm, or about 4 mm, or even less than about 4 mm, depending
on the overall size of the polarization converter 220. An unpolarized
light 250, such as the unpolarized combined light output 124 shown in
FIG. 1, enters the monolithic FEA 201, and exits the polarization
converter 220 as a first divergent p-polarized light 260b and a second
p-polarized light 260a. Generally, the path length of each polarization
state of unpolarized light 250 is essentially the same through the
optical element 200, as can be seen from the discussion that follows.

[0040] The first lenslet array 210 includes a representative first lens
212 of the plurality of lenses disposed to accept the unpolarized light
250 and output a convergent unpolarized light to a second lens 232 of the
second lenslet array 230 in the monolithic FEA 201. In some cases, each
lens of the first lenslet array 210 can be, for example, a cylindrical
lens, and can be arranged in an array such that the long axis of the
cylinder is perpendicular to the cross-section shown in FIG. 2. In some
cases, each lens of the first lenslet array 210 can be, for example, a
spherical lens, and can be arranged in a rectangular array. Each lens of
the first lenslet array 210 has a first optical axis 211, and a surface
214 that is typically a planar surface. The first lenslet array 210 can
be formed from a glass or a polymer, and can include a substrate
coincident with surface 214, or can instead be a monolithic lenslet array
formed from a single material.

[0041] The second lenslet array 230 includes a representative second lens
232 disposed such that the optical axis 211 of each lens of both the
first lenslet array 210 and the second lenslet array 230 are coincident,
and the unpolarized light 250 becomes a divergent unpolarized light shown
by representative first unpolarized light 252, second unpolarized light
254, and third unpolarized light 256. In some cases, each lens of the
second lenslet array 230 can be, for example, a cylindrical lens, and can
be arranged in an array such that the long axis of the cylinder is
perpendicular to the cross-section shown in FIG. 2. In some cases, each
lens of the second lenslet array 230 can be, for example, a spherical
lens, and can be arranged in a rectangular array. Each lens of the second
lenslet array 230 is aligned to the optical axis 211, and has surface 214
that is typically a planar surface. The second lenslet array 230 can be
formed from a glass or a polymer, and can include a substrate coincident
with surface 214, or can instead be a monolithic lenslet array formed
from a single material. Generally, the focal point of each lens (for
example, first lens 212) of the first lenslet array 210 is positioned at
the first principle plane of each lens (for example, second lens 232b) of
the second lenslet array 230. Generally, both the first lenslet array 210
and the second lenslet array 230 can be formed from a single material to
form monolithic FEA 201, as described elsewhere.

[0042] In some cases, a high index glass can be used for the lenslet
array. Also, high index glasses with lead tend to have low stress optical
component (SOC) that can lead to a preferable low-birefringence. However,
it can be difficult to mold small lens features into glass. As a result,
polymeric materials are preferred for the lenslet array construction,
including, for example, such polymers as polycarbonates (PC),
cyclo-olefin polymers (COP), cyclo-olefin co-polymers (COC, and
polymethylmethacrylates (PMMA). Exemplary polymeric materials include,
for example, cyclo-olefinic polymer materials such as Zeonex® (for
example, E48R, 330R, 340R, 480R, and the like, available from Zeon
Chemicals L.P., Louisville, Ky.); cyclo-olefin co-polymers such as
APL5514ML, APL5014DP and the like (available from Mitsui Chemicals, Inc.
JP); polymethylmethacrylate (PMMA) materials such as WF100 (available
from Mitsubishi Rayon Technologies, JP) and Acrypet® VH001 (available
from Guangzhou Hongsu Trading Co., Guangdong, CN); and polycarbonate,
polyester, or polyphenylene sulfide materials. Generally, a birefringence
of less than 50 nm, or less than 30 nm, or even less than 20 nm can be
preferred (at a nominal wavelength of 550 nm). However, a much wider
range of materials can be used, for example, higher birefringence
materials become acceptable, such as those having a birefringence of
about 50 nm or more, when the FEA homogenizing component is placed after
the illumination source and before the light is polarized, as described
elsewhere.

[0043] The polarization converter 220 is disposed to accept the divergent
unpolarized light, such as shown by representative first unpolarized
light 252, second unpolarized light 254, and third unpolarized light 256,
and output a divergent polarized light as described below. Polarization
converter 220 includes a first prism 222 having first and second faces
223 and 228, a second prism 224 having third and fourth faces 221 and
227, and a third prism 226 having second face 228 (common with first
prism 222), fifth face 225, and diagonal face 229. A reflective polarizer
240 is disposed on the diagonal between first and second prisms 222, 224.

[0044] The reflective polarizer 240 can be any known reflective polarizer
such as a MacNeille polarizer, a wire grid polarizer, a multilayer
optical film polarizer, or a circular polarizer such as a cholesteric
liquid crystal polarizer. According to one embodiment, a multilayer
optical film polarizer can be a preferred reflective polarizer.
Generally, reflective polarizer 240 can be a Cartesian reflective
polarizer or a non-Cartesian reflective polarizer. A non-Cartesian
reflective polarizer can include multilayer inorganic films such as those
produced by sequential deposition of inorganic dielectrics, such as a
MacNeille polarizer. A Cartesian reflective polarizer has a polarization
axis direction, and includes both wire-grid polarizers and polymeric
multilayer optical films such as can be produced by extrusion and
subsequent stretching of a multilayer polymeric laminate. In one
embodiment, reflective polarizer 240 is aligned so that one polarization
axis is parallel to a first polarization direction, and perpendicular to
a second polarization direction. In one embodiment, the first
polarization direction can be the s-polarization direction, and the
second polarization direction can be the p-polarization direction.

[0045] A Cartesian reflective polarizer film provides the polarizing beam
splitter with an ability to pass input light rays that are not fully
collimated, and that are divergent or skewed from a central light beam
axis. The Cartesian reflective polarizer film can comprise a polymeric
multilayer optical film that comprises multiple layers of dielectric or
polymeric material. Use of dielectric films can have the advantage of low
attenuation of light and high efficiency in passing light. The multilayer
optical film can comprise polymeric multilayer optical films such as
those described in U.S. Pat. No. 5,962,114 (Jonza et al.) or U.S. Pat.
No. 6,721,096 (Bruzzone et al.).

[0046] The polarization converter 220 further includes a polarization
rotating reflector that includes a quarter-wave retarder 242 and a
broadband mirror 244 disposed on fourth face 227. Polarization rotating
reflectors are discussed elsewhere, for example, in PCT Publication No.
WO2009/085856 (English et al.). The polarization rotating reflector
reverses the propagation direction of the light and alters the magnitude
of the polarization components, depending of the components and their
orientation in the polarization rotating reflector. The polarization
rotating reflector generally includes a reflector and a retarder. In one
embodiment, the reflector can be a broadband mirror that blocks the
transmission of light by reflection. The retarder can provide any desired
retardation, such as an eighth-wave retarder, a quarter-wave retarder,
and the like. In embodiments described herein, there can be an advantage
to using a quarter-wave retarder and an associated reflector. Linearly
polarized light is changed to circularly polarized light as it passes
through a quarter-wave retarder aligned at an angle of 45° to the
axis of light polarization. Reflections from the reflective polarizer and
quarter-wave retarder/reflectors result in efficient light output from
the polarization converter. In contrast, linearly polarized light is
changed to a polarization state partway between s-polarization and
p-polarization (either elliptical or linear) as it passes through other
retarders and orientations, and can result in a lower efficiency of the
polarization converter.

[0047] Preferably, quarter-wave retarder 242 includes a quarter-wave
polarization direction aligned at +/-45° to the first polarization
direction. In some embodiments, the quarter-wave polarization direction
can be aligned at any degree orientation to first polarization direction,
for example from 90° in a counter-clockwise direction to
90° in a clockwise direction. It can be advantageous to orient the
retarder at approximately +/-45° as described, since circularly
polarized light results when linearly polarized light passes through a
quarter-wave retarder so aligned to the polarization direction. Other
orientations of quarter-wave retarders can result in s-polarized light
not being fully transformed to p-polarized light, and p-polarized light
not being fully transformed to s-polarized light, upon reflection from
the mirrors, resulting in reduced efficiency as described elsewhere.

[0048] A second broadband mirror 246 is disposed adjacent the diagonal 229
of third prism 226. The components of the polarization converter
including prisms, reflective polarizers, quarter-wave retarders, mirrors
and any other components can be bonded together by a suitable optical
adhesive. The optical adhesive used to bond the components together can
have a lower index of refraction than the index of refraction of the
prisms used in the light combiner. A polarization converter that is fully
bonded together offers advantages including alignment stability during
assembly, handling and use.

[0049] According to one particular embodiment, the prism faces 221, 223,
225, 227, 229 are polished external surfaces that are in contact with a
material having an index of refraction "n1" that is less than the
index of refraction "n2" of prisms 222, 224, and 226. According to
another embodiment, all of the external faces of the polarization
converter 220 (including end faces, not shown) are polished faces that
provide TIR of oblique light rays within polarization converter 220. The
polished external surfaces are in contact with a material having an index
of refraction "n1" that is less than the index of refraction
"n2" of prisms 222, 224, and 226. TIR improves light utilization in
polarization converter 220, particularly when the light directed into the
polarization converter 220 is not collimated along a central axis, that
is the incoming light is either convergent or divergent.

[0050] Unpolarized light rays 250 coincident with the first optical axis
211 of the first lens 212 passes through monolithic FEA 201, becomes
first divergent unpolarized light ray 252, enters polarization converter
220 through third face 221 of second prism 224, and intercepts reflective
polarizer 240 where it is split into first p-polarized divergent light
ray 262 and first s-polarized divergent light ray 253. In a similar
manner, another of the unpolarized light rays 250 entering first lens 212
at a position separated from the first optical axis 211 passes through
monolithic FEA 201, becomes second divergent unpolarized light ray 254,
and is split into second p-polarized divergent light ray 264 and second
s-polarized divergent light ray 255. In yet another similar manner,
another of the unpolarized light rays 250 entering first lens 212 at a
second position separated from the first optical axis 211 passes through
monolithic FEA 201, becomes third convergent unpolarized light ray 256,
and is split into third p-polarized divergent light ray 266 and third
s-polarized divergent light ray 257.

[0052] First, second, and third s-polarized divergent light rays 253, 255,
257 reflect from reflective polarizer 240, exit second prism through
fourth face 227, change to circular polarized divergent light as they
pass through quarter-wave retarder 242, reflect from broadband mirror 244
changing the direction of circular polarization, and become fourth,
fifth, and sixth p-polarized divergent light 263, 265, 267, as they pass
again through quarter-wave retarder 242. Fourth, fifth, and sixth
p-polarized divergent light 263, 265, 267 pass through reflective
polarizer 240, exit polarization converter 220 through first face 223 of
first prism 222, and become second p-polarized divergent light 260a.
Second and first p-polarized divergent light 260a and 260b pass through
the remaining portions of the projection system described in FIG. 1, with
an improved uniformity.

[0053] In some cases, the quarter-wave retarder 242 can instead be
disposed adjacent reflective polarizer 240, between broadband mirror 244
and reflective polarizer 240 (not shown), and a similar optical path can
be traced through the polarization converter 220, as known to one of
skill in the art. In some cases, the polarization rotating reflector that
includes the quarter-wave retarder 242 and broadband mirror 244 can
instead be disposed on the third face 221, and the unpolarized input
light rays 250 can enter polarization converter 220 through fourth face
227, and a similar optical path can be traced through the polarization
converter 220, as known to one of skill in the art.

[0054] In one particular embodiment, minimizing the amount of birefringent
effects that can impact a beam of light traversing a Fly's Eye's Array
(FEA) includes selection of an FEA material that has a low stress optical
coefficient (SOC), and is thin. The low SOC manifests as low induced
birefringence in the substrate of the FEA after both surfaces of the
substrate have been structured/molded into matching lenslet arrays. A
second aspect to achieving low birefringence is to reduce the optical
path in the substrate material. This requires a short focal length design
for the lenslets. The focal point of the first lenslet array is cast onto
the principal plane of the second lenslet array. The short focal length
drives a small radius of curvature for each lenslet element. As a result,
the lateral size of each lenslet typically is reduced, in order to
maintain the aperture of each lenslet element (that is, no flat region of
the array, without power). Therefore, the resultant number of lenslets
per array is increased, which can improve beam homogenization.

[0055] Having a small lenslet lateral size requires a high precision in
the registration of the optical axis of each lenslet element in the first
lenslet array to the corresponding lenslet optical axis in the second
lenslet array. In one particular embodiment, for example, a FEA used in
an LED illuminator can have an approximately 0.6 mm×0.9 mm lenslet
aperture and with typical mechanical positional tolerances of 30-50 um,
the light crosstalk from the misalignment will be severe. The need for a
low birefringent FEA element drives small and thin lenslet element
design. A small lenslet element drives the need for a monolithic FEA
fabrication for maintaining the required alignment precision. A thin
lenslet substrate ensures little birefringence for the same amount of
stressed induced in the substrates.

[0056] FIG. 3 shows a side-view schematic of an optical element 400,
according to one aspect of the disclosure. Optical element 400 can be
used as the homogenizing polarization converter module 130 in the image
projector 100 as shown in FIG. 1. Optical element 400 includes a
polarization converter 420, a first lenslet array 410, and a second
lenslet array 430. Each of the first lenslet array 410 and the second
lenslet array 430 can be referred to as a "Fly-Eye Array", or FEA, as
known in the art. The first lenslet array 410 and the second lenslet
array 430 together form a monolithic FEA 401 that has a thickness "t",
and can include an optional central substrate 414 between first lenslet
array 410 and second lenslet array 430.

[0057] Each of the elements 410-446 shown in FIG. 3 correspond to
like-numbered elements 210-246 shown in FIG. 2, which have been described
previously. For example, third prism 426 of FIG. 3 corresponds to third
prism 226 of FIG. 2, and so on. In FIG. 3, the relative position of
reflective polarizer 440 has changed from the position of reflective
polarizer 240 in FIG. 2, and as a result, the path length of each
component of the unpolarized input light 450 is different in the
configuration shown in FIG. 3, as can be seen in the figure. Generally,
the path lengths of each polarization component are preferably the same;
however, the optical element 400 will function as an alternate embodiment
of a homogenizing polarization converter.

[0058] Unpolarized light rays 450 coincident with the first optical axis
411 of the first lens 412 passes through monolithic FEA 401, becomes
first divergent unpolarized light ray 452, enters polarization converter
420 through third prism face 421 of second prism 424, and intercepts
reflective polarizer 440 where it is split into first p-polarized
divergent light ray 462 and first s-polarized divergent light ray 453. In
a similar manner, another of the unpolarized light rays 450 entering
first lens 412 at a position separated from the first optical axis 411
passes through monolithic FEA 401, becomes second divergent unpolarized
light ray 454, and is split into second p-polarized divergent light ray
464 and second s-polarized divergent light ray 455. In yet another
similar manner, another of the unpolarized light rays 450 entering first
lens 412 at a second position separated from the first optical axis 411
passes through monolithic FEA 401, becomes third convergent unpolarized
light ray 456, and is split into third p-polarized divergent light ray
466 and third s-polarized divergent light ray 457.

[0060] First, second, and third s-polarized divergent light rays 453, 455,
457 reflect from reflective polarizer 440, exit second prism through
fourth prism face 427, and become first s-polarized divergent light 460a.
First and second s-polarized divergent light 460a and 460b pass through
the remaining portions of the projection system described in FIG. 1, with
an improved uniformity.

[0061] FIG. 4 shows a cross-section schematic of a polarization converter
520 according to one particular embodiment of the disclosure.
Polarization converter 520 can be used in place of any of the already
described polarization converters, for example, polarization converter
220 in optical element 200 and polarization converter 420 in optical
element 400. For brevity, the lenslet arrays have been removed from FIG.
4, and only the path of light through the polarization converter 520 will
be described. It is to be understood, however, that the polarization
converter module 130 of FIG. 1 includes polarization converter 520 and
any associated lenslet array, similar to those described in FIGS. 2-3.

[0062] Each of the elements 520-546 shown in FIG. 4 correspond to
like-numbered elements 220-246 shown in FIG. 2, which have been described
previously. For example, third prism 526 of FIG. 4 corresponds to third
prism 226 of FIG. 2, and so on. In FIG. 4, the relative position of
reflective polarizer 540 has changed from the position of reflective
polarizer 240 in FIG. 2, and as a result, the path length of each
component of the unpolarized input light 552 is different in the
configuration shown in FIG. 4, as can be seen in the figure. Generally,
the path lengths of each polarization component are preferably the same;
however, the polarization converter 520 will function as an alternate
embodiment of a homogenizing polarization converter.

[0063] In one particular embodiment shown in FIG. 4, the second prism 524
has an optional elongated portion "P" extending the length of prism face
523. The extended length of prism face 523 can serve to increase the path
length of the unpolarized input light 552, and as a result, the
homogenization of the unpolarized input light 552 as described, for
example, in co-pending U.S. Patent Application No. 61/292,574, entitled
"Compact Optical Integrator" (Attorney Docket No. 65902US002) filed on
Jan. 6, 2010.

[0064] In one particular embodiment, the polarization converter 520
includes a half-wave retarder 548 disposed between first prism 522 and
third prism 526 as shown in FIG. 4. In one particular embodiment, the
half-wave retarder 548 can instead be disposed adjacent the prism face
525, in a manner similar to the half-wave retarder 448 shown in FIG. 3.
In some cases, the half-wave retarder can be placed anywhere within the
optical path of the light transmitted through the reflective polarizer
540, such that the polarization state of the transmitted light is changed
to the polarization state of the reflected light. In one particular
embodiment, the half-wave retarder can be inserted adjacent to any of the
prism faces 523, 540, 548, 525, and 529.

[0066] Unless otherwise indicated, all numbers expressing feature sizes,
amounts, and physical properties used in the specification and claims are
to be understood as being modified by the term "about." Accordingly,
unless indicated to the contrary, the numerical parameters set forth in
the foregoing specification and attached claims are approximations that
can vary depending upon the desired properties sought to be obtained by
those skilled in the art utilizing the teachings disclosed herein.

[0067] All references and publications cited herein are expressly
incorporated herein by reference in their entirety into this disclosure,
except to the extent they may directly contradict this disclosure.
Although specific embodiments have been illustrated and described herein,
it will be appreciated by those of ordinary skill in the art that a
variety of alternate and/or equivalent implementations can be substituted
for the specific embodiments shown and described without departing from
the scope of the present disclosure. This application is intended to
cover any adaptations or variations of the specific embodiments discussed
herein. Therefore, it is intended that this disclosure be limited only by
the claims and the equivalents thereof.